BOSTON (CBS) — The Celtics will be without Al Horford on Wednesday night when they host the Los Angeles Lakers.

The C’s big man has entered the NBA’s concussion protocol, the team announced Wednesday afternoon. Horford was struck in the head by the forearm of Atlanta forward Kent Bazemore while going for a rebound in the second quarter of Boston’s win over the Hawks on Monday night, but did not experience any concussion symptoms until Wednesday morning.

Here is video of when Al Horford got hit against the Hawks. He will miss Wednesday's game against the Lakers as part of the NBA's concussion protocol pic.twitter.com/2eU06mhgnR

It’s unclear how long Horford will be sidelined, but he missed nine games at the beginning of last season after suffering a concussion in practice. Given this is his second concussion in as many seasons, he could be sidelined for a some time.

“You have an extra level of concern with anybody who has an injury to the head,” Celtics head coach Brad Stevens told 98.5 The Sports Hub’s Zolak & Bertrand shortly after Boston made the announcement on Wednesday. “I don’t know from a medical standpoint how much the concern increases as you have multiple concussions, but we’re very well aware of that. That’s why the protocol is set up within our organization and league, to make sure he is good and cleared before he returns to play.”

Horford has been incredible for the 9-2 Celtics, averaging 14.6 points and 9.2 rebounds per game. He’s having one of the best shooting seasons of his career, hitting 53 percent of his shots from the floor and 47 percent of his attempts from three-point range. His impact goes well beyond what you see on the stat sheet, as Horford’s spacing and screens have been a big part of Kyrie Irving’s early season success in his first year with the Celtics.

“We can’t replace him with one guy; we’ll have to replace him by committee,” Stevens said. “We play through him a lot, so we’ll have to have guys who make the next right play. Don’t try to do all that he does, but try to do what they do best to make up for what [Al] does.”